C++ Socket服务端/客户端程序：输入Exit无法触发终止逻辑问题

Hey there, let's dig into why your Exit command isn't working—and fix a few other potential headaches while we're at it!

The Root Cause: Comparing C-Strings Wrong #

The big mistake here is how you're checking for the "Exit" command. In C/C++, when you write strBuf == "Exit", you're not comparing the actual text of the strings—you're comparing the memory addresses of the two char pointers. The string literal "Exit" lives in a read-only section of memory, while strBuf is on the stack, so this condition will never be true, no matter what you type.

To compare the content of C-style strings correctly, you need to use the strcmp() function from <string.h>. It returns 0 when the two strings are identical.

Other Hidden Issues in Your Code #

While we're fixing the main problem, let's address a few other things that could cause trouble later:

• scanf("%s", strBuf) stops reading as soon as it hits a space or newline—so if you type a message with spaces, only the first word gets sent to clients.
• No thread exit logic: Even if you fixed the string comparison, the loop would keep running after sending the termination message.
• Buffer overflow risk: scanf doesn't check if input exceeds your 100000-byte buffer, which could lead to memory corruption.

Fixed Code Example #

#include <string.h>
#include <stdio.h>
#include <unistd.h>

// Assuming nClient and nFDList are properly defined elsewhere
int nClient;
int nFDList[];

void *admin_handler(void *ptr) {
    char strBuf[100000];
    const char strExitMsg[] = "Server: terminated.";
    const char strExitCmd[] = "Exit";

    while (1) {
        // Use fgets instead of scanf to read full lines (including spaces)
        if (fgets(strBuf, sizeof(strBuf), stdin) == NULL) {
            // Handle read error or EOF
            break;
        }

        // Remove the trailing newline added by fgets
        size_t msgLen = strlen(strBuf);
        if (msgLen > 0 && strBuf[msgLen - 1] == '\n') {
            strBuf[msgLen - 1] = '\0';
            msgLen--;
        }

        // Correct string comparison with strcmp
        if (strcmp(strBuf, strExitCmd) == 0) {
            // Send termination message to all clients
            for (int i = 0; i < nClient; i++) {
                write(nFDList[i], strExitMsg, strlen(strExitMsg) + 1);
                close(nFDList[i]); // Close the client connection
            }
            nClient = 0; // Reset client count
            break; // Exit the loop to end the thread cleanly
        } else {
            // Forward the message to all clients
            for (int i = 0; i < nClient; i++) {
                write(nFDList[i], strBuf, msgLen + 1);
            }
        }
    }
    return NULL;
}

Key Fixes Explained #

• strcmp for string comparison: This checks the actual characters in the strings, not their memory addresses.
• fgets instead of scanf: Reads entire lines, preserves spaces, and prevents buffer overflow by using sizeof(strBuf) to limit input length.
• Trim trailing newline: fgets includes the newline character from input—we remove it so clients don't get extra line breaks.
• Proper thread exit: After sending termination messages, we close client connections, reset the client count, and break out of the loop to end the thread cleanly.

Bonus Tips for C++ Development #

Since you're working in C++, consider using std::string instead of C-style char arrays. It's safer, supports direct == comparison for content, and handles memory management automatically. Also, make sure to add thread safety for nClient and nFDList (like a mutex) if clients can connect/disconnect while this thread is running—otherwise you might hit array out-of-bounds errors.

内容的提问来源于stack exchange，提问作者Andrew